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1. Field of the Invention
This invention relates to coherent imaging systems including, for example, radar, sonar, seismic, and ultrasound systems, using vibratory energy, and in particular, but not limited to, phased array ultrasound imaging systems for scan formats such as linear, steered linear, sector, circular, Vector(copyright), steered Vector(copyright) and other types of scan formats in imaging modes such as, by way of example only, B-mode (gray-scale imaging mode), F-mode (flow or color Doppler imaging mode), M-mode (motion mode) and D-mode (spectral Doppler mode). Although the invention will be discussed with respect to an ultrasound system, the invention can be implemented with other types of coherent imaging systems.
2. Background of the Invention
A. Literature
The open literature, which presents issues relevant to imaging systems in general, includes the following documents which are incorporated herein by reference:
1. Dan E. Dudgeon, xe2x80x9cFundamentals of Digital Array Processing,xe2x80x9d Proceedings of the IEEE, volume 65, pp. 899-904, June 1977.
2. Dan E. Dudgeon and Russell M. Mersereau, Multidimensional Digital Signal Processing, Chapter 6, Section 2: xe2x80x9cBeamforming,xe2x80x9d Prentice Hall, 1984.
3. William C. Knight, Roger G. Pridham, and Steven M. Kay, xe2x80x9cDigital Signal Processing for Sonar,xe2x80x9d Proceedings of the IEEE, volume 69, pages 1451-1506, November 1981. (Digital beamformers for use in sonar described on pages 1465-1471.)
4. Roger G. Pridham and Ronald A. Mucci, xe2x80x9cA Novel Approach to Digital Beamforming,xe2x80x9d Journal of the Acoustical Society of America, volume 63, pages 425-434, February 1978.
5. Roger G. Pridham and Ronald A. Mucci, xe2x80x9cDigital Interpolation Beamforming for Low-Pass and Bandpass Signals,xe2x80x9d Proceedings of the IEEE, volume 67, pages 904-919, June 1979.
6. P. Barton, xe2x80x9cDigital Beamforming for Radar,xe2x80x9d IEE Proceedings, volume 127, part F, number 4, August 1980.
7. P. D. Carl, G. S. Kino, C. S. Desilets and P. M. Grant, xe2x80x9cA Digital Synthetic Focus Acoustic Imaging System,xe2x80x9d Acoustic Imaging, volume 8, pp. 39-53, 1978.
8. B. D. Steinberg, xe2x80x9cDigital Beamforming in Ultrasound,xe2x80x9d IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, volume 39, pp. 716-721, November 1992.
9. Hans Steyskal, xe2x80x9cDigital Beamforming Antennas,xe2x80x9d Microwave Journal, volume 30, No. 1, pp. 107-124, January 1987.
10. R. E. Crochiere and L. R. Rabiner, xe2x80x9cMultirate Digital Signal Processing,xe2x80x9d Chapter 2, Prentice Hall, 1983.
B. Analog and Hybrid (Analog-Digital) Beamformer Systems
Relevant analog and hybrid (analog-digital) phased array beamformer system art can be found in the following patents which are incorporated herein by reference:
C. Digital Receive Beamformer Systems
The concept of a digital receive beamformer system has been proposed in the art with respect to ultrasound systems. By way of example, the following U.S. patents, all of which are incorporated herein by reference, discuss various aspects of such systems. The patents include:
The basic feature of a digital receive beamfomer system as disclosed above can include: (1) amplification of the ultrasound signal received at each element of an array such as, for example, a linear array; (2) direct per channel analog-to-digital conversion of the ultrasound signal with an analog-to-digital sampling rate at least twice the highest frequency in the signal; (3) a digital memory to provide delays for focusing; and (4) digital summation of the focused signals from all the channels. Other processing features of a receive beamformer system can include phase rotation of a receive signal on a channel-by-channel basis to provide fine focusing, amplitude scaling (apodization) to control the beam sidelobes, and digital filtering to control the bandwidth of the signal.
D. Transmit Beamforming
The above literature points out the ever-present desire to achieve more accurate focusing, better resolution, better sensitivities and higher frame rates in ultrasonic images. In order to do so, versatile adjustments of the beamforming characteristics are required in order to optimize the results for a given scan requirement. The greatest versatility is obtained when the ultrasound instrument can entirely change the number of beams transmitted simultaneously, the pulse waveform (PW) or continuous waveform (CW) characteristics, time delays and apodization values on a per-scan-line basis. However, such versatility can undesirably require extensive hardware resources if carried out in a direct implementation.
The above literature reveals extensive effort in the improvement of images through the use primarily of improved receive beamformers. Receive beamformers which employ digital techniques and digital signal processing have been reported in the prior art, though substantial improvements are still possible through innovative designs. Little effort, however, has been made to improve the characteristics of transmit beam formation. In the past, transmit beams were typically gated carrier pulses generated at a desired carrier frequency by analog circuitry. The only flexibility which was available to optimize the transmit pulse waveform shape (envelope) was typically an ability to specify the length of the pulse in terms of an integer number of carrier cycles it should contain, and some fixed analog filtering. Apodization and delay profiles for beamforming would be specified, and typically implemented in an analog fashion as well, with inherent precision limitations. The envelope shape of a pulse waveform was otherwise essentially fixed, and due to the limitations of analog processing, was not optimal. Also, whereas prior transmit beamformers were able to support different apodization profiles and different delay profiles for each firing in a scan, and were able to support different pulse lengths for each firing, the carrier frequency could not be changed between scan lines, nor could other characteristics of the envelope shape, other than its length, be modified.
There are significant advantages to be obtained by enhancing the flexibility of a transmit beamformer using digital processing techniques. For example, it would be desirable to be able to arbitrarily and independently shape the waveform which is to be applied to each of the ultrasonic transducer elements, in order to compensate for imperfections in the response of the transducer element or in the analog path to the transducer element. As another example, it would be desirable to change the waveform carrier frequency applied to each transducer element on a per-scan-line basis in order to mitigate the effects of grating lobes. See, for example, the above-cited METHOD AND APPARATUS FOR ADJUSTABLE FREQUENCY SCANNING IN ULTRASOUND IMAGING co-pending patent application. As yet another example, it would be desirable to improve focal precision in a transmit beam, such as by eliminating the tendency of analog components to drift over time or to correct for aberrating tissue (see, for example, the above-cited METHOD AND APPARATUS FOR REAL TIME, CONCURRENT ADAPTIVE FOCUSING IN AN ULTRASOUND BEAMFORMER IMAGING SYSTEM co-pending patent application). As still another example, it would be desirable to transmit pulses with special modulations, such as chirp or pseudo-random coded waveforms, in order to produce temporally longer pulses while maintaining range resolution in the resulting image. As yet another example, it would be desirable to be able to support multi-beam transmissions on a single firing as a way to increase the frame rate, to reduce speckle effects, or to achieve compound focusing (multiple focal points). As another example, it would be desirable to be able to transmit pulses having a precisely defined shape which will compensate for the distorting effects of attenuative body tissue. As yet another example, it would be desirable to update as many of the characteristics of a transmit pulse on a per-scan-line basis as possible. Simultaneous satisfaction of all these objectives cannot be obtained using presently available ultrasonic transmit beamformers.
Single channel digital programmable waveform generators are known in the field of test instruments for their ability to precisely generate arbitrary waveforms. See, for example, Priatko U.S. Pat. No. 4,881,190, and Tektronix, xe2x80x9cTest and Measurement Product Catalog 1949xe2x80x9d, pp. 337-359, both incorporated herein by reference. The techniques used in these test instruments are generally not applicable or practical for phased array ultrasonic transmit beamforming, however, and in any event, have not been used for that purpose. For example, they could support only a single transmit channel and could not perform beamforming, in part because of cost, power and space constraints.
As used herein, an xe2x80x9canalogxe2x80x9d signal is a signal whose value at any given moment in time can take on any value within a continuous range of values. Analog signals can also be continuous in time, or sampled in time. A xe2x80x9cdigitalxe2x80x9d signal, as the term is used herein, can take on only discrete values at discrete time intervals. Also as used herein, the term xe2x80x9cultrasonicxe2x80x9d refers to any frequency which is above the range of human hearing. Also as used herein, a device or function which is xe2x80x9cprogrammablexe2x80x9d includes those which can be programmed either by providing specific values for use by the device or function, or by selecting such values from a predetermined set of available values.
One way to achieve ultimate flexibility in ultrasonic digital transmit beamforming would be to write a digital representation of the entire waveform sample sequence or each transducer element, and each firing, into a memory. The waveform representations would be precomputed and stored to account for modulation, envelope shaping, and beamformation apodization and delays. A firing (transmit event) would then be effected by reading out of the memory the waveforms associated with all transducer elements simultaneously and applying them to digital-to-analog converters (DACs) associated with each respective transducer element, at the sample rate assumed by the waveform representation. Scanning would be effected by sequencing through the different waveform sets associated with each firing in the scan. Such a system would be a direct implementation of a digital transmit beamformer.
While such a system could be built and could achieve the desired objectives, it is impractical with current technology for several reasons. First, a large amount of memory would be required, and it would have to operate at very high data rates. Second, assuming an aspect of the desired flexibility includes the flexibility to alter the waveforms in real time prior to each firing, then the amount of computations required to compute each waveform, and the amount of time required to download all of the waveform samples into the memory, would reduce the scan frame rate to levels which are not diagnostically useful.
Preferably, therefore, roughly described, the present method and apparatus of the invention provides for a substantially digital signal processing architecture of independent transmitters, preferably each assigned to one or more transducers, which are fully programmable for adjustment of signal parameters and beamformation parameters at rates consistent with multiple range focusing and with updating at every scan line. Each transmitter has multiple processing channels that can support formation of multiple simultaneous beams (scan lines). The independence, programmability, and processor channelization support a versatility not available in prior art. The architecture achieves independent transmitters (1) by creating a separate central control apparatus (subject of co-pending patent application METHODS AND APPARATUS FOR FOCUS CONTROL OF TRANSMIT AND RECEIVE BEAMFORMER SYSTEMS) that determines all signal and beamformation parameters independent of all transmitters, and (2) by programming the parameters into each transmitter at rates needed to sustain multiple range focusing and scan-line-to-scan-line adjustments. The digital transmit beamformer architecture can therefore support conventional beamformation, and can also support enhanced transmit beamformer capability, such as adaptive focus beamformation. Signal and beamformation parameters that can be programmed on a scan-line interval basis include: delay sample values, apodization sample values, modulation frequency, signal-defining and signal-shaping filter values, gain, sample rate, gain and phase calibration adjustments, and number of simultaneous transmit beams. An advantage of a system architecture with independent transmitters having programmable features is the ability to support new transmit beamformation techniques, which can be accomplished by reprogramming the types of parameters sent to the transmitters.
The digital transmit beamformer system has a plurality of transmit processors each with a source of real- or complex-valued initial waveform samples of the ultimate desired waveform to be applied to one or more corresponding transducer elements. Preferably, for pulse wave (PW) transmissions, the waveform samples are a baseband (at or near 0 Hz) representation of the desired transmit pulse. In that case the waveform samples represent the real or complex envelope of the transmit pulse. The source of initial waveform samples might be a memory, for example, and it might be shared among two or more transmit processors. For continuous wave (CW) transmissions, each transmit processor provides a continual sequence of unit waveform samples. Each transmit processor applies beamformation delays and apodization to its respective initial waveform samples digitally, and digitally modulates the information to a carrier frequency. It also interpolates the information to the DAC sample frequency for conversion to an analog signal and application to the associated transducer element(s). Each transmit processor can process one to four transmit channels.
The digital transmit beamformer has a high degree of programmability. In particular, pulse shape can be programmed by assignment of appropriate initial waveform samples to each transmit processor; carrier frequency is also a programmable parameter, as are per-transmit-channel delay and apodization values. Arbitrary delay correction values, additional to the above delay values and computed by external equipment to adjust for aberration and calibration effects, can also be programmed for each transmit channel.
Because of the above parameterization of transmit waveforms, extensive flexibility is achieved without incurring the problems associated with the direct implementation approach described above. Additionally, the amount of information which must be specified to each transmitter in order to produce a desired composite beam response by firing the ultrasonic transducer array is substantially reduced compared to that of the direct implementation approach. Thus, for PW operation, pulse waveform parameters can be specified to the transmit beamformer on a per transmit channel and per firing basis, without degrading the scan frame rate to non-useful diagnostic levels. Waveform parameters can be specified to the transmitters by an external central control system (see above-cited METHOD AND APPARATUS FOR FOCUS CONTROL OF TRANSMIT AND RECEIVE BEAMFORMER SYSTEMS co-pending patent application) which is responsible for higher level flexibility, such as scan formats, focusing depths and fields of view; thus the parameterization permits each transmitter to concern itself with only the generation of a single output pulse waveform.
In another aspect of the invention, the transmit pulse delay which is specified for each transmit channel is applied in at least two components: a coarse delay which is an integer number of sample intervals and a focusing phase adjustment equivalent to a delay which represents a fractional sample interval. A xe2x80x9ccoarse delayxe2x80x9d is applied by time-delaying the memory read-out of initial waveform samples for the transmit channel by an integer number of samples of the memory read-out sample interval. Such time delays achieve a level of time coherence to within a single sample interval at the focal point(s) among the pulse waveforms produced by the different transmitters. Phase coherence is then achieved by phase rotation of each complex sample in a transmit channel by a carrier phase-angle-equivalent delay derived from the sample fraction portion of the transmit time delay specified to the transmitter as an integer-fraction value normalized to the sample interval. Additionally, if upsampling to the DAC sample frequency is accomplished in two stages instead of one, then more precise time coherence can be achieved. Specifically, an intermediate portion (referred to herein as the xe2x80x9cfine delayxe2x80x9d) of the transmit time delay which was specified to the transmit channel may be applied by additionally delaying the signal by an integer number of sample times at the intermediate sample rate. Note that as used herein, a delay can be either positive or negative, a negative delay being the same as a positive advance.
Each transmitter applies its specified apodization to the transmitter""s initial waveform samples rather than to the ultimate transmit waveform samples applied to the DAC, thereby permitting the apodization multiplier to operate at the initial waveform sample rate rather than at the much higher sample rate of the ultimate transmit waveform sample sequence. Similarly, complex phase rotation is performed preferably at the initial waveform sample rate rather than at the DAC sample rate.
The eventual transmit carrier frequency Fc out of the DAC can be specified to the transmit signal path, to any desired frequency within a substantially continuous predefined range of frequencies. The desired frequency Fc is defined by selecting one of a predefined plurality of available nominal center frequencies F0, and specifying a vernier factor v=Fc/F0. Each choice of F0 chooses a different set of digital filters in the signal path, optimized for the selected nominal center frequency F0. The value v can be specified to any value within a range of 0 to 2 and with a precision as fine as the number of bits with which it is specified, although in practice the specification may be limited to a smaller predefined range. The available F0 frequencies are preferably spaced closely enough such that, together with the vernier factors, the transmitter can produce any carrier frequency within a large range of frequencies, with a precision limited only by the number of bits with which v is specified. The transmitter preferably modulates the initial waveform samples by Fc by first applying a phase ramp, its slope determined by v, to each sample, and then modulating the signal by F0 using digital processing means. Application of the phase ramp is performed by phase rotation of each real- or complex-valued sample by the phase ramp value for that sample, again, preferably at the initial waveform sample rate.
In order to produce multiple beams, multiple waveforms (one associated with each beam) are produced simultaneously in each transmitter. The multiple waveforms, after application of appropriate delay and apodization, are superposed in the transmitter before being applied to the transducer element related to the transmitter. Waveform parameters for the different beams can be shared or specified separately for each waveform to be produced by a transmitter, including the initial waveform samples.
When producing multiple transmit beams, each transmitter preferably processes its multiple waveforms in an interleaved manner using shared processing resources to a point in the signal path at which they are summed to generate the composite waveform that will generate multiple beams when combined with the composite waveforms produced by the other transmitters. Prior to that point in the signal path, the transmitter is operable in any of several predefined processing modes. The available processing resources preferably define processing modes with different parameter sets in a computational efficiency trade-off among (1) the maximum number of beams; (2) the initial waveform sampling rate (related to the maximum transmit bandwidth); and (3) transmit frequency (more precisely, the nominal center frequency F0). The trade-offs provided by the available processing modes permit maximum usage of the available computational capacity of the hardware.